1. Field of the Invention
This invention relates to bearing devices used in various kinds of motors.
2. Related Art
Spindle motors having the kind of structure shown in FIG. 14 are widely known and are used for example for floppy disk drive (FDD) motors. In FIG. 14, reference numeral 21 denotes a spindle which is the rotating shaft of the motor, and this spindle 21 is rotatably mounted in a bearing device 30. A hub plate 22 and a rotor yoke 23 are fixed to the spindle and rotate as one with the spindle 21, and a disk-shaped drive magnet 25 is fitted to the rotor yoke 23. The bearing device 30 is mounted on a stator base 27, and a coil 29 is fitted to the portion of the stator base 27 that faces the drive magnet 25.
FDD motors are increasingly required to be thin, and in this respect facing surface constructions such as that shown in FIG. 14 are good. However, because in facing surface type motors the thrust load is greater than the radial load, compared to facing edge type, the bearing device 30 consists of two different types of bearing: a ball bearing (roller bearing) 33, which bears the thrust load; and a sintered, oil impregnated bearing 32, which bears the radial load.
Especially in spindle motors used in FDDs, it requires that the hub plate 22 has to be a precision part. If the hub plate 22 is not sufficiently well balanced it will shake, and the clearance between the head and the disk will be varied because the disk loaded onto it will also shake. For this reason, in a spindle motor, the bearing structure is very important.
However, the following problems are associated with conventional bearing devices:
(1) Because the axial dimension of the sintered & oil impregnated bearing 32 is short, high precision cannot be ensured. PA1 (2) Because a ball bearing 33 is used, some lubrication is necessary, and since it is not possible to supply oil to the bearing, wear and hence seizure occur readily, and bearing life is short. PA1 (3) The use of two different types of bearing, i.e. the ball bearing 33 and the sintered & oil impregnated bearing 32, raises the cost of the motor and, because space is required, makes it difficult to reduce the overall diameter of the motor. PA1 In view of the foregoing problem, an object of the present invention is to provide a bearing device which is of high precision, which has a long life, and which is low in cost.